The invention relates to a method for controlling the cross-directional profile of a reeling nip. The invention also relates to an apparatus for implementing the aforementioned method.
In the final end of a machine manufacturing paper, paperboard, soft tissue or the like or a finishing apparatus for paper, paperboard or soft tissue or the like, a paper web which is typically several meters wide and which has been produced and/or treated in the preceding machine sections, is reeled around a reeling shaft, i.e. a reel spool to form a machine reel. In this reeling up process a reeling cylinder that is bearing-mounted rotatable is typically used for guiding the paper web on the machine reel, wherein the nip contact between the reeling cylinder and the machine reel is utilized to influence the quality of the reel produced thereby. The ends of the reel spool are affected by means of a suitable loading mechanism to adjust the nip contact between the machine reel that is being formed and the reeling cylinder. Such reeling concepts and loading methods related thereto are disclosed, for example, in the Finnish patent 91383 and in the corresponding U.S. Pat. No. 5,251,835, as well as in the Finnish patent application 950274 and in the corresponding U.S. Pat. No. 5,690,298.
The measurement of the cross-directional profile of such a reeler is disclosed for example in the U.S. Pat. No. 5,048,353 in which one or several sensors operating on piezoelectric principle have been installed on the surface layer of the reeling cylinder, said sensors reacting to the pressure prevailing in the nip. The sensors have been installed spirally around the length of the reeling cylinder so that they measure the cross-directional profile of the pressure prevailing in the reeling nip.
In addition, the publication EP-860391 discloses a reeler, in which the web is guided on a reel via a supporting member formed of several endless belts or wires arranged next to each other in the longitudinal direction of the guide roll, said supporting member being passed via the guide rolls. Thus, by means of the belt loops it is possible to attain a long reeling nip having an even pressure in the area of the lower half of the reel. The aim is to control the nip pressure of the reeling nip through the tension of individual belt loops. Thus, each belt loop requires separate belt tensioning means. According to the publication, it is possible to profile the nip pressure on the basis of the measured tension of individual belt loops. It is a problem in this solution that because the supporting member is composed of several belt loops arranged next to each other in the longitudinal direction of the guide roll, it is difficult to monitor the condition of the belts, and maintain and repair them. Furthermore, it is difficult to control the rotation speed of separate belt loops, and it requires separate controlling means. It is also difficult to hold the belts moving in the machine direction in their correct locations in the longitudinal direction of the guide rolls so that they do not drift on top of each other. Furthermore, the separate belt tensioning means required by each belt loop causes lack of space in the surroundings of the reeler.
Furthermore, the WO publication 98/55384 discloses a reel-up in which the reeling nip is formed by means of a loop of a supporting member and a reel spool. The total tension of the belt is controlled by means of load cells attached to a guide roll guiding the belt. The total tension of the belt thus attained is also used for controlling the nip pressure of the reeling nip.
Both when using a conventional reeler based on a reeling cylinder and a belt reeler utilizing a supporting member according to the above-mentioned EP publication 860391 and WO publication 98/55384 there is a basic problem in the reeling process: it is difficult to get an even cross-directional profile in the machine reel that is being produced. Consequently, the irregularities produced in the reeling, such as creases caused by the slackness of the belt, and local dents caused by excessive tension of the web, transfer to the customer rolls. In the above-mentioned publications attempts have been made to solve this problem by means of controlling the cross-directional linear pressure of the reeling nip. This is, however, difficult, because the controlling requires accurate measurement results. The solutions shown in the publications EP860391 and WO 98/55384 are based on the controlling of the nip pressure of the reeling nip through the total tension of the belt. This is not a sufficiently accurate method to eliminate the problems in the reeling.